METAZOAN INTERACTIONS WITH MICROBIAL COMMUNITIES IN CAMBRIAN THROMBOLITES

The evolution of metazoan grazers and burrowers has played a major role in shaping the microbial ecology throughout the Phanerozoic. Thrombolite fabrics preserve a record of ancient microbial bioactivity as well as any burrowing or grazing that shaped them. Distinguishing between the growth structures of these thrombolites and features shaped by metazoan disruption is an important approach to investigating the complexity of the thrombolite fabrics and microbial-metazoan interactions. The Great Basin Cambrian thrombolites in California preserve a variety of fabric classes. Some can be linked to simple microbial growth structure and fabric that has been influenced by metazoan burrowing. However, others are sufficiently complicated that is unclear in hand specimen samples whether their fabrics reflect growth structure, metazoan influences, or both. To better understand how thrombolites reflect ancient ecosystems, we used a serial sectioning technique to scan around 150 slices of eight thrombolite samples. These images were aligned and displayed in 3D visualization software which was run in the KeckCAVES virtual reality facility at UC Davis. By filtering the sample components using a color density variation tool, each component was displayed individually which helped to better understand its morphology. Some samples from the Nopah Formation that reflect only growth structures in hand specimen revealed evidence of possible metazoan grazing on the microbial matrix in 3D, which was not possible to interpret in 2D hand specimen. Additional details of the size and shape of metazoan burrows were identified in other samples, and some show evidence of at least two microbial fabrics in a single sample with different morphology and chemistry. These preliminary results demonstrate that 3D techniques will allow us to interpret complicated classes of thrombolite fabrics that could not be correlated to microbial growth activity or metazoan disruption. We will continue to use this 3D technique to test hypotheses related to multiple microbial growth stages, which could have produced two or more microbial matrices, as well as the impact of metazoan influences at various stages of their growth.